Wireless communication systems are widely spread all over the world to provide various types of communication services such as voice or data. The wireless communication system is designed for the purpose of providing reliable communication to a plurality of users irrespective of their locations and mobility. However, a wireless channel has an abnormal characteristic such as path loss, noise, fading due to multipath, an inter-symbol interference (ISI), the Doppler effect due to mobility of a user equipment, etc. Therefore, various techniques have been developed to overcome the abnormal characteristic of the wireless channel and to increase reliability of wireless communication.
Multiple Input Multiple Output (MIMO) is a technique for supporting reliable high-speed data services. The MIMO technique improves data transmission/reception efficiency by using multiple transmit (Tx) antennas and multiple receive (Rx) antennas. Examples of the MIMO technique include spatial multiplexing, transmit diversity, beamforming, etc.
Multiple receive antennas and multiple transmit antennas form a MIMO channel matrix. A rank can be found from the MIMO channel matrix. The rank is the number of spatial layers. The rank may also be defined as the number of spatial streams which can be transmitted at the same time by a transmitter. The rank is also referred to as a spatial multiplexing ratio. Assuming that the number of transmit antennas is Nt and the number of receive antennas is Nr, the rank R is R≦min{Nt, Nr}.
In general, a wireless communication system is a single carrier system supporting a single carrier. The transmission rate is proportional to transmission bandwidth. Therefore, for supporting a high-speed transmission rate, transmission bandwidth shall be increased. However, except for some areas of the world, it is difficult to allocate frequencies of wide bandwidths. For effectively using fragmented small frequency bands, a spectrum aggregation technique is being developed. The spectrum aggregation technique may be also referred to as bandwidth aggregation or carrier aggregation. The spectrum aggregation technique is to obtain the same effect as if which a frequency band of a logically wide bandwidth may be used by aggregating a plurality of physically contiguous or non-contiguous frequency bands in a frequency domain. Through the spectrum aggregation technique, multiple carriers can be supported in the wireless communication system. The wireless communication system supporting multi-carrier is referred to as a multi-carrier system. The carrier may be also referred to as a radio frequency (RF), component carrier (CC), etc.
Meanwhile, as a mobile communication system of a next generation (i.e., post-3rd generation), an international mobile telecommunication-advanced (IMT-A) system is standardized in an international telecommunication union (ITU). The purposes of IMT-A system are to provide a high-speed transmission rate of 1 gigabits per second (Gbps) in downlink communication and 500 megabits per second (Mbps) in uplink communication and to support an Internet protocol (IP)-based seamless multimedia service. In a 3rd generation partnership project (3GPP), a long term evolution-advanced (LTE-A) system is considered as a candidate technique for the IMT-A system.
In an LTE system, downlink transmission is supported for up to 4 transmit antennas, and uplink transmission is supported only for 1 antenna. In an LTE-A system, downlink transmission is considered to support up to 8 transmit antennas, and uplink transmission is considered to support multiple antennas. The LTE system is a single carrier system supporting one carrier. The LTE-A system is a multiple carrier system supporting multiple carriers.
An advanced system (e.g., an LTE-A system) having an increased number of transmit antennas in each of downlink transmission and uplink transmission, as described above, as compared with an existing system (e.g., an LTE system) can be taken into consideration. Further, the advanced system may be a system supporting an increased number of component carriers as compared with the existing system. With an increase in the number of transmit antennas and the number of component carriers, the transmission rate of information can be increased.
Although the advanced system is evolved to increase a completion level of the existing system, it is desirable that the advanced system should be designed to maintain backward compatibility with the existing system. This is because the provisioning of compatibility between the advanced system and the existing system is advantageous in terms of user convenience, and is also advantageous for a service provider since existing equipment can be reused.
For the purpose of compatibility with the existing system, different system modes, such as an advanced system mode and an existing system mode, may be defined in the advanced system. Accordingly, there is a need for a method and apparatus for efficiently transmitting and receiving signals in an advanced wireless communication system.